A circuit of the general type described above is disclosed in the journal "Electronic Design News" of Mar. 21, 1985, pages 205 to 220, where it is diagrammed in FIG. 7 on page 216. In addition to the band pass filters provided for the possible single tones, it also contains the same number of additional band pass filters tuned to the first harmonic of the mid-band frequency of a band pass filter for each individual single tone. These additional band pass filters in combination with an additional analyzer circuit provided for them serve to increase the speech certainty in recognizing of two-tone multifrequency signals. When a single tone together with its first harmonic is recognized, then it can be assumed with greater certainty that this single tone is invalid, i.e., it can be attributed to the transmission of speech frequencies, because the two-tone compounds signals are transmitted in the form of the purest possible sinusoidal tones so they are almost free of harmonics.
The circuit described above thus needs twice the number of band pass filters in comparison with the number of possible single tones plus twice the analyzer circuits to recognize the two-tone compound signals. When embodied in digital technology, the scanning frequency required for operation must have at least four times the value of the highest single-tone frequency or at least twice the value of the highest harmonic frequency that occurs. This requires very short computation times in signal analysis, so this in turn limits the number of possible transmission channels a computer can analyze.